The present invention relates time tracking in a communications system, and more particularly to time tracking in a synchronous communications system. Even more particularly, the present invention relates to determining a timing error, or clock phase error, of a clock signal based on a comparison between an incoming signal and a Taylor series of an expected signal.
A great deal has been published in the area of symbol time tracking. Namely, schemes employing early and late samples, oversampling, and interpolation have been used to adjust a timing error, or clock phase error, in a clock circuit at a receiver. The need for such symbol time tracking schemes arises due to the fact that incoming radio frequency signals may be slightly delayed or advanced as a result of distortions that occur within a communications channel between a transmitter and the receiver, of relative physical movement between the transmitter and the receiver, or of clock frequency mismatch between the transmitter and the receiver. As a result of such distortions, the clock circuit within the receiver must be adjusted so that it accurately tracks the timing of the incoming radio frequency signal. Systems and methods are therefore needed in order to make timing adjustments to the clock circuit so as to account for these distortions.
One approach to making these timing adjustments to the clock circuit utilizes three sets of samples that are taken from the incoming radio frequency signal. The first of these sets of samples is taken early, i.e., before the clock circuit indicates that such samples should be taken. A second set of such samples is taken on time, i.e., when the clock circuit indicates that such samples should be taken. And, a third set of samples are taken late, i.e., after the clock circuit indicates that such samples should be taken. After the three sets of samples are taken from the radio frequency signal, a comparison is made between a portion of each of the sets of samples, which is taken during a synch pattern portion of the incoming signal, and a representation of what these samples are expected to be. The synch pattern portion of the incoming signal contains a known pattern of information, and therefore should match the expected set of samples, after account is made for attenuation that occurs within the communications channel.
Based on the comparison, when the on time set of samples is highly correlated with the expected set of samples, the clock circuit is accurately tracking the incoming radio frequency signal. However, in the event the early set of samples is better correlated with the expected samples, a positive timing offset, or phase shift, is applied to the clock signal generated by the clock circuit so as to advance the timing of the clock signal. Similarly, in the event the late set of samples better correlates with the expected set of samples, a negative timing offset, or phase shift, is applied to the clock signal generated by the clock circuit so as to retard the timing of the clock signal. Thus, a timing offset adjustment is made to the clock signal, whenever the expected set of samples better correlates with the early or late set of samples. As a result, the clock signal will more accurately track the incoming radio frequency signal over time.
Early-late-type symbol time tracking provides for good adjustment of the clock signal based on variations in timing in the radio frequency signal. Unfortunately however, this approach requires that three sets of samples be taken from the radio frequency signal and that three separate correlations (comparisons) be performed between each of the sets of samples taken from the radio frequency signal and the expected set of samples. This additional sampling and correlating requires increased processing time and overhead at the receiver and limits the selection of prefabricated receiver hardware from which the receiver can be fabricated to that hardware which permits acquisition of the three sets of samples. Problematically, not all commercially available prefabricated receiver hardware makes available both on time samples and early and late samples. Therefore, improvements are needed in systems and methods for symbol time tracking in receivers.
Another approach to symbol time tracking utilizes oversampling, which involves sampling at a much higher rate than is needed for accurate symbol acquisition, i.e., a much higher rate than the Nyquist rate. Correlation techniques similar to those employed with early-late schemes are employed in oversampling schemes in order to select which of the several sets of samples provides the most appropriate representation of the incoming radio frequency signal.
A further approach to symbol time tracking is to sample the incoming radio frequency signal at the Nyquist rate and then to interpolate so as to generate off time samples, analogous to early and late samples. The interpolation approach thus involves not only performing correlations between the expected samples, and the interpolated samples but requires that the interpolated samples be generated.
An even further approach to symbol time tracking can be used when raw samples are passed through a filter, e.g., a matched filter, before symbol time tracking is performed. In this approach, time-offset reversions of the filter are used to generate early and late filtered samples from the raw samples. Unfortunately, this approach requires multiple filtering operations followed by the three sets of correlations required in the early-late symbol time tracking approach, described above.
Thus, the oversampling, interpolation, and filtered approaches to symbol time tracking suffer from the problems associated with the above-described early-late symbol time tracking scheme, namely, they require that additional samples be taken and/or that additional computations be performed. Thus, improvements are needed in systems and methods for symbol time tracking within a receiver.
The present invention advantageously addresses the above and other needs.